The present invention relates to a multi-slice X-ray CT apparatus and a method of controlling the same for obtaining a plurality of tomographic images of a subject using X-rays.
Generally, in an X-ray CT apparatus for reconstructing a plurality of slice images, a plurality of detectors for detecting an X-ray beam are arranged along the circumference of a xe2x80x9cgantryxe2x80x9d (each row comprising the plurality of detectors is referred to as an X-ray detector array), and a plurality of the detector arrays are arranged side-by-side in a direction orthogonal to the circumference. The X-ray CT apparatus combines signals detected by the X-ray detector arrays depending upon a specified thickness (slice thickness) to obtain n (four in general) signals in a slice direction.
Recently, there is a need to obtain X-ray tomographic images with different thicknesses by such an X-ray CT apparatus, and in response to the need, the following technique has been developed.
FIG. 1(A) is a diagram illustrating the relationship between a cross section across the plurality of X-ray detector arrays and an X-ray tube. Each X-ray detector array comprises thousands of detector elements along the circumference of the gantry, and the drawing is a cross-sectional view of an i-th channel of the X-ray detector arrays. A total of sixteen X-ray detector arrays (sixteen rows) are shown.
For the sake of simplification, the width of a detector array is assumed to be 1 mm, and the detector arrays are designated as A, B, . . . , P from the left in the drawing.
In this configuration, signals obtained by a group of four consecutive X-ray detector arrays A-D are additively combined by a multiplexer (i.e., a data acquisition section), and signals obtained by groups of the X-ray detector arrays E-H, I-L and M-P are each additively combined in a similar manner. Thus, four X-ray tomographic images each having a thickness of 4 mm can be obtained. Alternatively, by changing the combination of signals, i.e., by combining signals from six adjacent X-ray detector arrays, for example, three X-ray tomographic images each having a thickness of 6 mm can be obtained. In some cases, it is at least possible to obtain X-ray tomographic images having thicknesses of 4 mm, 6 mm, 3 mm, and 3 mm. Such combining is achieved by the multiplexer.
There is another technique in which an open/close controllable collimator is disposed proximate to the X-ray tube, as shown in FIG. 1(B). As shown, the detector comprises two X-ray detector arrays A and B, and the opening of the collimator is controlled to obtain signals with slice thicknesses depending upon the size of the opening.
In the former case (FIG. 1(A)), the multiplexer generates a signal with a slice thickness according to the required combination. Since the multiplexer has a total of sixteen input signals, there are very many combinations for obtaining four X-ray tomographic images by selecting desired signals from the signals, or for obtaining four X-ray tomographic images by combining a plurality of signals. Therefore, the internal configuration of the multiplexer is inevitably highly complicated, and in practice, the multiplexer is configured to handle only a limited number of combinations.
In the latter case, although the slice thicknesses of X-ray tomographic images are variable by opening or closing the collimator, the number of obtainable slices is currently limited to two, and more than two slices cannot be obtained.
It might be attempted to obtain a plurality of X-ray tomographic images having different slice thicknesses by combining the multiplicity of X-ray detector arrays shown in FIG. 1(A) and the collimator control technique shown in FIG. 1(B). However, the number of combinations theoretically depends upon the number of X-ray detector arrays employed, and the configuration of the multiplexer would not be simplified after all.
The present invention is directed to solving such problems, and provides a multi-slice X-ray CT apparatus and a method of controlling the same which enables a plurality of X-ray tomographic images having a plurality of practical slice thicknesses to be reconstructed by a small number of X-ray detector arrays.
For this purpose, the present invention provides a multi-slice X-ray CT apparatus provided with an X-ray generator and an X-ray detector disposed facing each other with a subject placed therebetween, the X-ray detector having a plurality of detector arrays for detecting X-rays from the X-ray generator, the apparatus rotating the X-ray generator and the X-ray detector to construct X-ray tomographic images having a plurality of slice thicknesses in a direction of conveying the subject, the apparatus comprising a collimator for forming a slit defining a range of X-rays generated by the X-ray detector impinging upon the X-ray detector, and collimator regulating means for regulating the width of the slit corresponding to the direction of conveying the subject, wherein not all the widths of the X-ray detector arrays in the X-ray detector in the direction of conveying the subject are the same.
In accordance with a preferred embodiment of the present invention, the number of X-ray detector arrays in the X-ray detector is four, and the widths of two outer detector arrays in the conveying direction is larger than the widths of two center detector arrays. In this case, it is preferable that the widths of the two center detector arrays be the same, and the widths of the two outer detector arrays be the same. As a result, configuration of the detector section can be simplified, and signals with varied slice thicknesses can be extracted.
Moreover, the collimator regulating means preferably further comprises means for regulating the position of the collimator in the Z-axis direction. As a result, signals for an additional number of slice thicknesses can be extracted.
As can be seen from the above description, and according to the present invention, the total number of X-ray detector arrays can be relatively reduced, and X-ray tomographic images having practical and varied slice thicknesses can be obtained while simplifying configuration.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.